1. Technical Field of the Invention
The present invention relates to a semiconductor device having a circuit configured with thin-film transistors (hereinafter, referred to as TFTs) and a method for manufacturing same. For example, the invention relates to an electronic apparatus mounting, as a part, an electro-optical device represented by a liquid-crystal display or a light-emitting device having OLEDs.
Incidentally, the semiconductor device in this description refers to a device as a whole which is capable of functioning by the utilization of a semiconductor characteristic, i.e. electro-optical devices, semiconductor circuits and electronic apparatuses are all fallen under semiconductor devices.
2. Description of the Related Art
Recently, attentions are drawn to the art to structure a thin-film transistor (TFT) by the use of a semiconductor film (thickness: approximately several to several hundred nm) formed over a substrate having an insulating surface. Thin-film transistors are broadly used on electronic devices, such as ICs and electro-optical devices. Particularly, development is hurried up for a switching element for an image display device.
Conventionally, the liquid-crystal display is known as an image display device. There is a tendency of frequent use of the liquid-crystal display of the active-matrix type because of the capability of obtaining an image with definition as compared to the passive-type liquid-crystal display devices. In the active-matrix liquid-crystal display device, a display pattern is formed on the screen by driving the pixel electrodes arranged in a matrix form. Specifically, by applying a voltage to between a selected pixel electrode and a counter electrode corresponding to that electrode, optical modulation is done in the liquid-crystal layer arranged between the pixel electrode and the counter electrode. The optical modulation is recognized as a display pattern by the observer.
Meanwhile, for the light-emitting devices using OLEDs, TFTs are requisite elements in realizing active-matrix drive scheme. Consequently, the light-emitting device using OLEDs has, on each pixel, at least a TFT functioning as a switching element and a TFT supplying current to the OLED. Light-emitting elements using an organic compound as phosphors, featured in small thickness, light weight, high responsibility, direct-current low voltage drive, are expected for the application to the next-generation display panel. In particular, the display device arranging the light-emitting elements in a matrix form is considered excellent in respect of its wide viewing angle and hence higher visibility as compared to the conventional liquid-crystal display device.
The luminescent mechanism of a light-emitting element is considered as follows. That is, by applying a voltage to a pair of electrodes sandwiching an organic compound layer, the electrons injected at the cathode and the holes injected at the anode are recombined at luminous centers in the organic compound layer. The molecular exciter, upon returning to the ground state, gives off energy-causing light emission. The excitation state is known as singlet excitation and triplet excitation. Electro-luminescence is considered possible through any of the excitation states.
For the light-emitting device formed by such light-emitting elements arranged in a matrix form, it is possible to use drive schemes of passive-matrix drive (simple matrix type) and active-matrix drive (active-matrix type). However, in the case with increased pixel density, the active-matrix type having a switch on each pixel (or one dot) is considered advantageous because of the capability of driving at low voltage.
There are increasing applications of such active-matrix type display devices (representatively, liquid-crystal and light-emitting display devices). With the increasing area in screen size, there are increasing requirements for improving definition, opening ratio and reliability. At the same time, requirements are toward production increase and cost reduction.
Conventionally, where TFTs are fabricated by using aluminum as a TFT gate interconnection material, there encounter projections, such as hillocks or whiskers, formed by thermal process or TFT poor operation or TFT characteristic reduction due to the diffusion of aluminum atoms into the channel region. To cope with this, in the case of using a metal material resistive to the thermal process, representatively high melt-point metal element, the problem arises that interconnection resistance increases or so with increase in screen area size, incurring increased power consumption.
Accordingly, it is a problem of the present invention to provide a structure of a semiconductor device that realizes low power consumption even where increased in screen size, and a method for manufacturing the same.